The present invention relates to a method of producing an Agxe2x80x94ZnO electric contact material.
Conventionally, Agxe2x80x94ZnO electric contact materials have been known to have considerably low contact resistance, but also to have unsatisfactory welding resistance and wear resistance. Therefore, enhancement of welding resistance and wear resistance of Agxe2x80x94ZnO electric contact materials is technically important for employment of such materials in make-and-break contacts, such as relays and switches, which are required to possess particularly excellent welding resistance and wear resistance.
A basic approach for enhancing welding resistance and wear resistance of Agxe2x80x94ZnO electric contact materials resides in uniformly dispersing ZnO micrograms in Ag. In order to attain uniform dispersion of ZnO micrograms, a variety of techniques have been proposed in the fields of powder metallurgy and internal oxidation, in relation to methods of producing Agxe2x80x94ZnO electric contact materials.
In powder metallurgy, powdered Ag and ZnO are mixed, and the mixture is shaped and sintered. Thus, reduction in the particle size of the powders to be mixed and sufficient mixing of ZnO micrograms result in a certain degree of dispersion. However, in powder metallurgy, the dispersion state of ZnO depends on the particle size of Ag powder and ZnO powder, and, therefore, the target uniformity in the dispersion state of ZnO grains of smaller size is considered to be limited. In addition, since Ag and ZnO have poor sinterability, voids are possibly formed in the produced sintered material, thereby lowering welding resistance and wear resistance in some cases. Thus, make-and-break contacts having highly satisfactory characteristics have never been produced from Agxe2x80x94ZnO material. Furthermore, powder metallurgy is not economically preferred for producing Agxe2x80x94ZnO material, in view of generally high production costs.
In internal oxidation, a predetermined amount of an Agxe2x80x94Zn alloy is sequentially cast, rolled, blanked, and cut, to thereby produce an alloy product of specific shape. The product is heated in an oxidizing atmosphere, to thereby selectively oxidize Zn in the Agxe2x80x94Zn alloy, causing dispersion of ZnO in Ag. As disclosed in Japanese Patent Publication (kokoku) No. 57-13613, dispersion of ZnO micrograms is attained through internal oxidation concomitant with addition of a third metallic element which causes dispersion of ZnO micrograms.
When ZnO micrograms are dispersed through internal oxidation concomitant with addition of a third metallic element, the ZnO micrograms dispersed in Ag tend to become acicular, and in many cases the acicular oxide is deposited in a streak-like manner. Dispersion becomes more distinct with increasing Zn content. Because this differs from the case of spherical ZnO micrograms dispersed through powder metallurgy, the acicular oxide deposited in a streak-like manner insufficiently enhances welding resistance and wear resistance. In addition, since the third metallic element added to disperse micrograms may affect the characteristics of Agxe2x80x94ZnO electric contact material, depending on the amount of addition, the amount of the third element to uniformly disperse ZnO micrograms is considered to be limited when conventionally-employed internal oxidation is carried out.
On the basis of the features describe above, Agxe2x80x94ZnO electric contact materials produced through powder metallurgy have often been employed. However, problems; e.g., controlling of powder particles and sinterability, still remain in the production of Agxe2x80x94ZnO electric contact material, even when powder metallurgy as described above is employed. In addition, at present, reducing production costs thereof is demanded.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide a method of producing an Agxe2x80x94ZnO electric contact material, which method can more uniformly disperse, in Ag, ZnO grains of smaller grain size; maintain low contact resistance of the contact material; enhance welding resistance and wear resistance; and produce the material at reasonable cost.
In order to solve the aforementioned problems, the present inventors have improved a method of producing an Agxe2x80x94ZnO electric contact material including internal oxidation, and have achieved production of an Agxe2x80x94ZnO electric contact material in which ZnO micrograms are uniformly dispersed at a level which had never before been attained. Accordingly, the invention provides a method of producing an Agxe2x80x94ZnO electrical contact material which comprises casting a predetermined amount of Ag and Zn and subjecting the resultant Agxe2x80x94ZnO alloy to internal oxidation to disperse ZnO in Ag, the method being characterized in that an Agxe2x80x94Zn alloy comprising 5-10 wt. % (as reduced to weight of metal) Zn, the balance being Ag, is formed into chips thereof; the chips are subjected to internal oxidation; the internally oxidized chips are compacted to thereby form billets; the billets are pressed and sintered; and subsequently, the sintered billets are extruded. The present inventors have found that this method can effect highly uniform dispersion, in Ag, of ZnO micrograms.
When the cast Agxe2x80x94Zn alloy is formed into chips for carrying out internal oxidation, the chips are compacted into billets, and the billets are pressed and sintered. The deposited ZnO assumes a streak-like dispersion state. However, when the billets are further extruded, the streak-like dispersion state of ZnO is converted to a uniform dispersion state of ZnO micrograms. The present inventors assume that the phenomenon occurs due to good wettability of ZnO to Ag.
When billets are formed into material such as wire rods through extrusion, a large shear stress is imposed on the billets in the longitudinal direction during deformation. The deformation during extrusion induces shear of ZnO dispersed in the billets in a streak-like manner, thereby yielding dispersion of ZnO micrograms in Ag. The present inventors have confirmed that a uniform dispersion state of oxide micrograms as yielded in the Agxe2x80x94ZnO electric contact material of the present invention cannot be attained in an Agxe2x80x94SnO2 electric contact material; i.e., a material containing an oxide of poor wettability to Ag. SnO2 that is an oxide having poor wettability to Ag cannot be formed into micrograms even though a large amount of shear stress is applied to a billet in the longitudinal direction during extrusion. In contrast, ZnO that is an oxide having good wettability to Ag is subjected to shear stress concomitant with deformation of Ag when a large amount of shear stress is applied to the billet in the longitudinal direction during extrusion. Thus, ZnO deposited in a streak-like manner in the billet is further fractured to form micrograms thereof, thereby yielding a very uniform dispersion state of ZnO micrograms to an extent which has never before been attained.
In order to produce the Agxe2x80x94ZnO electric contact material of the present invention, particularly, the following process conditions must be satisfied. One condition concerns pressing and sintering to which the billets produced by compacting internally oxidized chips are subjected. The pressing and sintering must be carried out until residual voids and defects in the billets disappear. For example, pressing and sintering of the billets must be performed repeatedly, to thereby sufficiently remove voids and defects in the billets.
The other process condition concerns extrusion which is carried out as a final process. Extrusion must be carried out to a relatively large extrusion ratio. Preferably, the extrusion ratio of the surface area of a billet to that of a produced rod is controlled to 51:1 or higher. The reason for such a high extrusion ratio is that ZnO contained in Ag can be considerably uniformly dispersed in the form of ZnO micrograms by employment of the ratio, thereby enhancing production yield. Typical extruders have an extrusion capacity; i.e., an achievable extrusion ratio, of approximately 350:1. In the method of producing an Agxe2x80x94ZnO electric contact material, such a high extrusion ratio can also be employed.
The method of the present invention provides an Agxe2x80x94ZnO electric contact material having a uniform dispersion state of ZnO micrograms in Ag which has never before been attained through a conventional internal oxidation method. Therefore, the material maintains low contact resistance thereof and exhibits enhanced welding resistance and wear resistance. The method of the present invention can produce an Agxe2x80x94ZnO electric contact material at production costs lower than those involved in powder metallurgy, and the produced Agxe2x80x94ZnO electric contact material has characteristics approximately equal to those of an Agxe2x80x94ZnO electric contact material produced through powder metallurgy.
In the method of producing an Agxe2x80x94ZnO electric contact material of the present invention, when the starting alloy contains only Ag and Zn, an alloy comprising 5-10 wt. % Zn, the balance being Ag, is preferred. When the Zn content is less than 5%, welding resistance and wear resistance cannot be enhanced to a practical level; whereas when the Zn content is in excess of 10%, internal oxidation of the alloy becomes difficult. Even though the alloy is internally oxidized, contact resistance increases considerably and processability of the alloy is degraded.
The present inventors have conducted extensive studies on the aforementioned method of producing an Agxe2x80x94ZnO electric contact material, and have found that employment of an Agxe2x80x94Znxe2x80x94Cu alloy or an Agxe2x80x94Znxe2x80x94Cuxe2x80x94Ni alloy as a starting alloy yields an Agxe2x80x94ZnO electric contact material having more excellent characteristics.
When the aforementioned Agxe2x80x94ZnO electric contact material is produced from an Agxe2x80x94Znxe2x80x94Cu alloy, ZnO micrograms are also uniformly dispersed in Ag. Furthermore, the uniform dispersion state of ZnO micrograms in Ag which is attained in the presence of Cu enhances the contact resistance of the produced electric contact material as compared with a similar dispersion in Ag of only ZnO.
The present inventors have confirmed that when an Agxe2x80x94ZnO electric contact material produced from only Ag and Zn serving as metallic components is formed into make-and-break contacts, ZnO film is deposited on the contacts after repetition of making and breaking at AC 250V and 10A, thereby elevating contact resistance. Through observation of the contact surface, deposition of layer-like ZnO is recognized at portions damaged by arcing. The inventors have elucidated that the ZnO deposits cause elevated contact resistance.
However, the method of the present invention employing additional Cu provides an Agxe2x80x94ZnO electric contact material in which increase in contact resistance during making and breaking caused by ZnO is effectively prevented. The assumed mechanism is that Cu forms a solid solution with ZnO and micrograms of the solid solution are uniformly dispersed in Ag. Briefly, Cu which forms a solid solution with ZnO prevents formation of ZnO film on the contacts during making and breaking of the contacts.
The method of the present invention employing additional Cu provides an Agxe2x80x94ZnO electric contact material which maintains considerably low contact resistance and exhibits excellent welding resistance and wear resistance. The produced electric contact material has a practically sufficient level of characteristics under load conditions of approximately AC 250V and 10A where generally-used relays and switches can operate.
In the method of producing an Agxe2x80x94ZnO electric contact material of the present invention, when Cu is added to the starting Agxe2x80x94Zn alloy, an alloy comprising 5-10 wt. % Zn and 0.01-3.00 wt. % Cu. the balance being Ag, is preferred. More preferably, the Ag alloy comprises 7-9 wt. % Zn and 0.20-0.50 wt. % Cu, in that addition of Cu is most effective.
When the Zn content is less than 5 wt. %, welding resistance and wear resistance cannot be enhanced to a practical level; whereas when the Zn content is in excess of 10 wt. %, internal oxidation of the alloy becomes difficult, thereby failing to yield uniform dispersion of ZnO micrograms even in the presence of Cu. In addition, even though a uniform dispersion state of ZnO micrograms is attained, the Zn content in excess of 10 wt. % makes maintenance of a practically low level of contact resistance difficult and reduces processability of the material. When the Cu content is less than 0.01 wt. %, the effect of Cu addition on reduction of the size of ZnO grains is weakened; whereas when the Cu content is in excess of 3.00 wt. %, Cu contained in the ZnO solid solution is readily segregated, thereby depositing CuO on the contacts and elevating contact resistance.
The method of the present invention employing an Agxe2x80x94Znxe2x80x94Cuxe2x80x94Ni alloy as a starting alloy provides an Agxe2x80x94ZnO electric contact material which exhibits more enhanced wear resistance when it is formed into make-and-break contacts.
In general, Ni is known as an additive element for depositing ZnO micrograms during production of an Agxe2x80x94ZnO electric contact material through internal oxidation. In contrast, the present inventors have confirmed through their research that no particular difference is observed in effect on depositing ZnO micrograms between an Agxe2x80x94ZnO electric contact material containing Cu and that containing Ni and Cu. However, when the starting alloy contains Ni, the produced electric contact material exhibits enhanced wear resistance under load conditions of approximately AC 250V and 10A where generally-used relays and switches can operate. The assumed mechanism for enhancement of wear resistance is that Ni partially forms a solid solution with ZnO and micrograms of the Ni-containing oxide are uniformly dispersed in Ag.
In the method of producing an Agxe2x80x94ZnO electric contact material of the present invention, when Cu and Ni are added to the starting Agxe2x80x94Zn alloy, an alloy comprising 5-10 wt. % Zn, 0.01-3.00 wt. % Cu, and 0.01-0.50 wt. % Ni, the balance being Ag, is preferred. More preferably, the Ag alloy comprises 7-9 wt. % Zn, 0.20-0.50 wt. % Cu, and 0.05-0.20 wt. % Ni, in that the concomitant effect of ZnO, Cu, and Ni attains the optimum balance.
When the Ni content is less than 0.01 wt. %, wear resistance is not effectively enhanced; whereas when the Ni content is in excess of 0.50 wt. %, Ni segregates in the Ag alloy before undergoing internal oxidation, and NiO coarse grains are generated after having undergone internal oxidation. The thus-formed NiO coarse grains cause an increase in contact resistance. In this case, Fe or Co may be used instead of Ni, because Fe and Co also contribute to the enhancement of wear resistance to an extent similar to that attainable by Ni. The Zn content range and the Ni content range are similar to those described above, and repeated description thereof is omitted.
As described hereinabove, an Agxe2x80x94ZnO electric contact material produced through the method of the present invention has a uniform dispersion state of ZnO micrograms in Ag which has never been attained through a conventional internal oxidation method. Therefore, the material maintains low contact resistance and exhibits enhanced welding resistance and wear resistance.